The present application relates to a positive electrode active material which is used for a nonaqueous electrolyte secondary battery. In particular, the present application relates to a method for manufacturing a positive electrode active material capable of suppressing the generation of a gas in the inside of a battery and a positive electrode active material.
In recent years, because of conspicuous development of portable electronic technologies, electronic appliances such as mobile phones and laptop personal computers have been recognized as a fundamental technology for supporting the highly computerized society. Also, research and development regarding high functionalization of these electronic appliances are being energetically advanced, and power consumption of these electronic appliances increases steadily in proportion thereto. On the other hand, these electronic appliances are required to be driven for a long period of time, and densification of high energy of a secondary battery which is a drive power source has been inevitably desired.
Also, from the viewpoints of occupied volume and mass of a battery to be built in an electronic appliance, it is desired that the energy density of the battery is as high as possible. At present, a lithium ion secondary battery is built in almost all of appliances because it has an excellent energy density.
As a positive electrode material for lithium ion secondary batteries, a lithium-containing transition metal compound capable of intercalating and deintercalating a lithium ion, or a complex oxide obtained by substituting a part of such a metal element is used. Also, LiMn2O4 having a spinel structure is widely used because it has a high energy density and a high voltage.
For example, a lithium ion secondary battery uses lithium cobalt oxide for a positive electrode and a carbon material for a negative electrode, respectively and is used at an operating voltage in the range of from 2.5 V to 4.2 V. In a unit cell, the fact that a terminal voltage can be increased to 4.2 V largely relies upon excellent electrochemical stability of a nonaqueous electrolyte material, a separator and so on.
Now, in existing lithium ion secondary batteries which operate at 4.2 V at maximum, a positive electrode active material to be used for a positive electrode, such as lithium cobalt oxide, applies a capacity of merely about 60% relative to its theoretical capacity. The lithium ion second battery is desirable to realize high energy density, high reliability and long life. As a method for enhancing these characteristics, especially an energy density of the battery, there is exemplified a method for setting up an upper voltage of charge high.
For achieving this, for example, as disclosed in WO 03/019713, it is theoretically possible to apply the residual capacity by furthering increasing the charge voltage. Actually, it is known that a high energy density is realized by setting up a voltage at the time of charge at 4.25 V or more. When the charge voltage is increased, it is possible to realize a high capacity because a larger amount of lithium is deintercalated and intercalated from a lithium complex oxide which is a positive electrode active material. It is theoretically possible to apply the residual capacity by increasing the charge voltage.
Above of all, lithium transition metal complex oxides composed mainly of nickel (Ni) or cobalt (Co), such as LixNiO2 (0<x≦1.0) and LixCoO2 (0<x≦1.0) are the most promising from the standpoints of high potential, stability and long life. Of these, positive electrode active materials composed mainly of lithium nickel oxide (LiNiO2) are a positive electrode active material displaying a relatively high potential, have a high charge current capacity and are expected to increase the energy density.
On the other hand, as described previously, in batteries in which the battery voltage is higher than that of existing secondary batteries, a charge and discharge cycle life is lowered, or a high-temperature characteristic is deteriorated. For example, it is considered that lithium nickel complex oxides such as lithium nickel oxide (LiNiO2) or lithium nickel complex oxides obtained by substituting a part of Ni with Co or Mn are high in stability at a high potential as compared with lithium nickel oxide (LiNiO2). However, such lithium nickel complex oxides are disadvantageous for increasing the energy density because their discharge potential or volume density is lowered as compared with lithium nickel oxide (LiNiO2).
Then, in order to stabilize the positive electrode active material composed mainly of lithium nickel oxide (LiNiO2), as disclosed in JP-A-2004-303591, it is proposed to allow a different kind of element, for example, aluminum (Al), magnesium (Mg), zirconium (Zr), titanium (Ti) or the like, to form a solid solution.
Also, as disclosed in JP-A-2000-164214, a construction in which LiNiO2 is used upon being mixed with a small amount of LiMn1/3Co1/3Ni1/3O2 or the like is proposed. Also, as disclosed in JP-A-2002-151078, it is proposed that the surface of lithium cobalt oxide is subjected to surface coating with spinel lithium manganese oxide, spinel lithium titanium oxide or a nickel cobalt complex oxide. Furthermore, as disclosed in JP-A-10-199530, it is proposed to contrive to stabilize active materials by heat treating lithium nickel cobalt oxide in an inert gas without relying upon substitution or coating with a metal element.